This invention relates to photographic reproduction and more particularly to a new electrophotographic element and to an electrophotographic process utilizing the new electrophotographic element. The electrophotographic process is a process in which an electrostatic latent image is produced by utilizing the property of photoconduction, i.e., variable conductivity dependent on the intensity of illumination. In the process an electrophotographic element is given a surface charge. The surface is then selectively exposed to light whereupon the surface charge dissipates from the surface at varying rates depending upon the intensity of illumination. The dissipation of the charge results from the conductive properties of the exposed photoconductor compound. The electrostatic latent image (nonvisible electrostatic charge pattern) may be produced in a conventional exposure operation, for example, by means of a lens-projected image or by contact-printing techniques.
The latent image so produced is developed, i.e., rendered visible, by means of a triboelectric powder or liquid toner. The triboelectric powder consists of a visible pigment and a fusable resinous compound. The triboelectric powder collects in the image areas and is fused into place by heating the resinous component of the powder. In developing an image with liquid toner, the liquid toner, which contains charged particles and a drying oil component, is washed over the surface of the exposed electrophotographic element. The charged particles are either attracted to or repelled by the latent image areas, depending upon the charge on the particles and the charge on the surface of the electrophotographic element and are bonded to the surface of the element as the drying oil component polymerizes. The surface of the electrophotographic element can be charged either positively or negatively and the toner can be charged either positively or negatively thereby giving four combinations of surface charge and toner charge to be utilized in producing the imaged product.
In electrophotographic processes the electrostatic latent image is commonly formed on the surface of a photoconductive insulating layer carried on a support. For example, material comprising such support and photoconductive layer is charged either positively or negatively by conventional means such as a corona discharge or the like to produce a uniform surface charge on the photoconductive support. Due to the substantial insulating character, i.e., the low conductivity of the insulating layer in the dark, the charge is retained by the photoconductive support. The photoconductive element is then exposed through an image bearing transparency or by reflex. This exposure activates the photoconductive compound thereby increasing the conductivity of the insulating layer in the exposed areas and allowing the charge to dissipate in these areas. The extent of this dissipation is directly proportional to the intensity of illumination or exposure. Thus, the charge in the activated or exposed areas is dissipated while the unexposed areas remain charged. The charge density at any point within the charge pattern is proportional to the intensity of illumination to which that point is subjected during the exposure. Thus, electrostatic latent images may consist of a pattern of varying charge densities depending upon the exposure conditions. This pattern of varying charge densities is rendered visible through the use of variously charged powders or toners as previously described.
The electrophotographic processes have become of increasing importance in recent years, especially in connection with office duplicating processes. Consequently, much interest has been aroused and great effort expended to obtain suitable materials for preparing the support and photoconductive insulating layers used in such copying processes.
Particular efforts have been expended to develop colorless organic photoconductors as a white copy sheet is generally preferred. Problems, however, have been encountered in obtaining colorless photoconductors that are sufficiently sensitive to be imaged using known light sources without, at the same time, being severely discolored by the interaction of light and oxygen.
It is, therefore, the major objective of the present invention to provide a new group of colorless organic photoconductors that do not become appreciably discolored when exposed to light and oxygen. The invention is also directed to a new electrophotographic material which is quickly and easily imaged by exposure to commonly employed light sources. Furthermore, the photoconductors of the present invention can be practically employed at low concentrations as opposed to other known photoconductors which, while they may exhibit electrophotographic responses at low concentrations, cannot practically be employed in photoconductive elements at the same low concentrations.